Field of the Invention
The present invention relates to an X-ray photographing system having a DSA (digital subtraction angiography) function, and particularly, to an X-ray photographing system capable of correctly catching the flow of a contrast medium in blood vessels of a patient irrespective of artifacts.
Description of the Prior Art
An angiography is widely used to diagnose and treat a patient having trouble in circulatory organs. The angiography employs a catheter or a guide wire to inject a contrast medium into patient's blood vessels. The injected contrast medium is photographed by X-ray camera, and the picked up X-ray images are used to diagnose and treat the patient. When photographing a large area such as one of the legs of a patient, or when inserting a catheter up to a target part of a patient, it is necessary to move the X-ray camera or a bed on which the patient is laid, so that a contrast medium or the head of the catheter is always photographed or viewed. The X-ray camera or the bed may be manually moved by an operator. Such manual method is frequently employed when inserting a catheter into a patient. There is also an automatic method.
FIG. 1 shows a conventional X-ray photographing system as an example of the automatic method. In the figure, the system picks up images of blood vessels of one of the legs of a patient. The system periodically moves an X-ray tube 101 in one direction from a position a to a position b, and from the position b to a position c.
FIG. 2 shows changes in images picked up by the system of FIG. 1.
This system automatically periodically moves the X-ray tube 101 with no regard to the flow of a contrast medium in the blood vessels. This system, therefore, will not always correctly provide images of a required region.
FIGS. 3 and 4 show images picked up by an X-ray photographing system that automatically moves a bed on which a patient is laid, to more correctly photograph a required region of the patient or the flow of a contrast medium in the patient.
The system photographs the patient at different time points without moving the bed at first, to provide live images L1, L2, and L3. These images are subtracted from one another to provide a temporal differential image T12 based on the images L1 and L2, a temporal differential image T23 based on the images L2 and L3, and so on. In each of the temporal differential images, points M having a highest concentration level are detected as leading ends of a contrast medium in blood vessels. If these points M are located below a predetermined line S, the bed with the patient is longitudinally moved for a predetermined distance. These processes are repeated according to successively obtained live images L4, L5, and so on, to thereby automatically move the bed with the patient. (This technique is disclosed in Japanese Laid-Open Patent Publication No. 3-53772.)
If an artifact (a virtual image) appears below the line S, the conventional X-ray photographing system may recognize the artifact as a point having the highest concentration, to improperly move the bed in response to the artifact before the actual leading ends of the contrast medium reach the line S. If the bed is improperly moved, the patient must be again photographed to elongate an inspection time and increase the quantity of X rays to which the patient is exposed.
Moreover, images of required regions may become out of sight because the bed is moved only longitudinally according to the conventional systems.